(1) Field of the Invention
This invention relates to a composition to be added to synthetic rubber to give the rubber improved tack. More particularly, this invention relates a tackifying composition for rubber which is a lignin-saponified tall oil pitch composite and to the process for making the composite.
(2) Description of the Prior Art
Styrene-butadiene (SBR) has a relatively low polarity compared to other synthetic rubbers; unlike natural rubber, SBR does not develop surface peroxidal activity upon mastication. Therefore, SBR has relatively poor inherent or processed tack. An adhesive is said to possess tack if, under the condition of application, only light pressure is required to produce a bond sufficiently strong to require work to restore the interface to its original separated state. Poor tack in SBR can be overcome by adding tackifiers, such as resins or natural rubber. The obvious drawback to using natural rubber, of course, is that any improvement realized is attained at the expense of using the very product which the polymer is intended to replace.
It has been known for several years that incorporation of small amounts of a copolymerizable unsaturated fatty acid monomer with styrene or styrene and butadiene improves the adhesive properties of the resulting polymer (i.e., Netherlands Application No. 6,411,493, Apr. 15, 1965--acrylic acid and fumaric acid and German Offenlegungschrift No. 2,437,365, Feb. 13, 1975,--itaconic).
The prior art includes many references to improving the properties of synthetic rubber by the incorporation therein of tall oil products. Four main products are usually obtained when refining crude tall oil distillation, namely, fore distillate (5-10%), tall oil fatty acids and a small amount of rosin-containing oils (30-35%), tall oil rosin (30-35%), and as a distillation residue, tall oil pitch (20-30%). Only the second and the third products are considered valuable raw materials and used by the chemical industry. In many distilleries at least the bulk of the fore distillate and the pitch is burned.
U.S. Pat. No. 2,657,147 to Van Valkenburgh discloses adding a mixture of tall oil acids and aniline to aid the compounding and vulcanization of rubber.
U.S. Pat. No. 3,474,059 to Body teaches that the adhesive properties of elastomeric olefin polymers can be improved by the incorporation therein of a small amount of tackifier additive comprised of at least two specific rosin-base materials.
In U.S. Pat. No. 3,649,580 to Arlt et al. the tack of an ethylene-propylene terpolymer rubber was improved by the addition of 1% to 20% tall oil pitch containing 80% to 100% non-volatile material as a 40.degree. C. to 90.degree. C. solution in rubber processing oil. Thus, dark brown tall oil pitch was distilled in a Pfaulder wiped film evaporator at 275.degree. C. and 0.5-1.0 mm. of mercury pressure until all volatile material was removed to give a tackifier of specific hardness.
U.S. Pat. No. 3,873, 482 to Severson et al. discloses a process by which commercial tall oil materials, a mixture of fatty acids and resin acids, are pyrolized in a hot tube to yield products which when added to a synthetic rubber composition improve its tack
U.S. Pat. No. 3,632,855 relates to a composition formed by the reaction of one mole of rosin acid with one mole of fatty olefin expoxide to give a hydroxy ester, useful as a tackifier in SBR rubber.
There have been processes proposed to improve the low quality, semifluid, tarlike, non-distillable tall oil pitch whereby it can be transformed into one or more products suitable for industrial applications.
For example, U.S. Pat. No. 3,926,936 discloses a process for manufacturing fatty acid and rosin, or a mixture of both, plus hard pitch of a new type from tall oil pitch.
U.S. Pat. No. 3,943,117 to C. G. Force relates to a process for saponifying tall oil pitch and for hydrolyzing the fatty acid esters found in a tall oil pitch fraction using small amounts of a cationic amine catalyst at specific conditions.
Finally, in 19 Izv. Vyssh. Uchebn. Zaved., Lesn, Zh., 1976, 113-116 (Russ.) the optimum saponification conditions of tall oil pitch, which give the highest yield of phytosterol, were determined by simplex analysis.
Copending U.S. Patent Application Ser. No. 106,569 to Force discloses a styrene-butadiene copolymer rubber including a saponified tall oil pitch as a tackifier.
Lignin-rubber coprecipitates as ordinarily produced according to prior art techniques, generally have the nature of a paste or mud which is difficult to process to obtain a dry cake or crumb, thus requiring high temperatures or long drying times and the need to be filtered. The particles of the coprecipitate, due to their very small size, tend to fill the pores of any filter medium, resulting in very low filtration and washing rates. When filtered and washed, the particles do not possess much cohesiveness and yield a crumbly filter cake which is difficult to handle during subsequent drying operations. The particles in the filter cake are non-comprehensible and retain large quantities of water which cannot be expressed by mechanical means and must be removed by application of heat. In general, the solids content of coprecipitates will range from 24% to 35%.
Several methods for overcoming the disadvantages of coprecipitating lignin with rubber have been proposed. In U.S. Pat. No. 3,167,523, Dimitri disclosed a method to improve the processing of modified lignin-rubber coprecipitates by heating a slurry prior to filtration. U.S. Pat. No. 3,312,643 to Ball discloses another process for treating lignin-rubber coprecipitates which include mixing formaldehyde with a slurry of lignin-rubber particles, and heating to cause the lignin and formaldehyde to react. U.S. Pat. No. 3,697,497 to Falkehag discloses still another process wherein hexamethylene tetramine modifications of lignin are used to make a high melting, high surface area lignin by a relatively simple, straight forward reaction which includes spray drying. U.S. Pat. No. 3,991,022 to Dimitri discloses a process for making lignin reinforced polymers comprising coprecipitating hexamethylene tetramine modified alkali lignin and rubber or rubber latices with a gas.
Among the other patents which teach the addition of a lignin material to rubber includes U.S. Pat. No. 1,217,157, to Coughlin, which discloses a rubber composition containing sulfite waste liquor solids. Another patent is U.S. Pat. No. 4,025,711 to Davidson et al. which describes a process for the addition of a lignin compound to styrene-conjugated diolefin polymers.
The general object of the invention is to provide synthetic rubber compositions having improved tack.
Another object of this invention is to provide a composite for synthetic rubber which has improved tack which comprises an alkali lignin and saponified tall oil pitch.
Still another object of this invention is to provide a process for making the coprecipitated alkali lignin composites.
Other objects, features and advantages of this invention will become evident from the foregoing detailed description of the invention.